The subject matter of the present invention relates to a well logging computer software which corrects an induction log for dip and well deviation effects.
It is important to the oil and gas industry to know the nature and characteristics of the various sub-surface formations penetrated by a borehole because the mere creation of a borehole (typically by drilling) usually does not provide sufficient information concerning the existence, depth location, quantity, etc., of oil and gas trapped in the formations. Various electrical techniques have been employed in the past to determine this information about the formations. One such technique commonly used is induction logging. Induction logging measures the resistivity (or its inverse, conductivity) of the formation by first inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring a phase component signal in a receiver signal generated by the presence of the eddy currents. Variations in the magnitude of the eddy currents in response to variations in formation conductivity are reflected as variations in the receiver signal. Thus in general, the magnitude of a phase component of the receiver signal, that component in-phase with the transmitter signal, is indicative of the conductivity of the formation.
U.S. Pat. Nos. 3,340,464; 3,147,429; 3,179,879; 3,056,917 and 4,472,684 are illustrative of typical prior-art well logging tools which utilize the basic principles of induction logging.
In each of the tools disclosed in these patents, a signal generator operates to produce an AC transmitter signal which is applied to a transmitter coil. The current in the transmitter coil induces in the formations a magnetic field, which, in turn, causes eddy currents to flow in the formations. Because of the presence of these formation currents, the magnetic field of the transmitter is coupled into a receiver coil R thereby generating a receiver signal (Logging tools having "a receiver coil" and "a transmitter coil" each of which may be comprised of one or more coils arranged in a predetermined geometrical fashion to obtain a desired response are commonly used). The receiver signal is then generally amplified and applied to one or more phase sensitive detectors (PSDs). Each PSD detects a phase component signal having the same phase as a phase reference signal which is also applied to the detector. The phase reference signal has a predetermined phase relationship to the current in the transmitter coil(s). The output of the PSD(s) may be further processed downhole, or may be sent uphole to surface equipment for processing or display to an operating engineer. Such processing may be accomplished using many well known techniques, including phasor deconvolution processing taught by U.S. Pat. No. 4,513,376 issued to T. Barber on Apr. 23, 1985; and U.S. Pat. No. 4,471,436 issued to R. Schaefer and T. Barber on Sept. 11, 1984. The disclosure of U.S. Pat. No. 4,471,436 is incorporated by reference into this specification. The disclosure of U.S. Pat. No. 4,513,376 is set forth below in APPENDIX A of this specification.
Since the earliest patents pertaining to focussed coil systems for induction logging (U.S. Pat. Nos. 2,582,314 and 2,582,315) the art has attempted to reduce the contribution to the tool response made by that part of the formation invaded by the drilling fluid ("invaded zone") and by formations above and below the region of interest. For example, U.S. Pat. No. 3,067,383, issued to D. R. Tanguy on Dec. 4, 1962 and incorporated herein by reference, discloses a sonde (hereinafter referred to as the Tanguy sonde) that has been very widely used in the industry and
U.S. Pat. No. 2,790,138 issued to A. Poupon on Apr. 23, 1957 discloses the use of a plurality of electrically independent transmitter-receiver pairs arranged symmetrically about the same center point. The response of that tool is obtained by combining the response of the several electrically independent pairs, these pairs being arranged in such a manner that contributions to the tool response from formation regions lying above or below the outermost coils and from the formation region close to the borehole are reduced.
Other patents (such as U.S. Pat. No. 3,329,889 issued to D. R. Tanguy on Jul. 4, 1967 and U.S. Pat. No. 3,453,530 issued to G. Attali on Jul. 1, 1969), incorporated herein by reference, have described induction tools that combine two or more focussed arrays in one tool with the object of making measurements in radially different parts of the formation. The deep array in these tools is derived from the deep-reading Tanguy sonde, with a derivative array having several receivers and sharing common transmitters with the deep array forming an array with a medium depth of investigation. The deep and medium arrays from these references will be referred to as ID and IM, respectively.
Induction logs produced by the above referenced art are interpreted through algorithms and models that assume azimuthal symmetry about the borehole. In reality, either formation dip or well deviation often destroys this symmetry.
FIG. 1 shows the assumed symmetry for most induction modeling. The bedding planes are perpendicular to the axis of the induction sonde, and the induced current in the formation does not cross any of the bed boundaries.
FIG. 2 shows the dip case. When the borehole, and thus the induction sonde, is not perpendicular to the bedding planes, the formation currents are forced to cross the bed boundaries. The response of the induction tool to the formation layers in the presence of dip has been studied in the literature. .sup.1,2,3
In FIG. 2a, the angle between the borehole and a line perpendicular to the bedding planes is called apparent dip, and can be caused by dip, well deviation, or a combination of the two. All further references to dip will imply apparent dip.
FIG. 3 shows the effect of dip on the deep induction array (ID) with traditional processing (ILD log) in a 50:1 contrast resistive bed. The logs do not show much change in the maximum resistivity level in the resistive bed with increasing dip because the large error is dominated by shoulder effect (shoulder effect refers to the tendency of induction arrays to include beds above and below the zone of interest into the measurement. This leads to measurements that are in error in resistive beds). The effect of apparent dip on logs with other environmental effects corrected (such as Phasor logs) is different from the traditional logs.
FIG. 4 shows the deep induction array (ID) processed using the Phasor method (deep induction array Phasor, or IDPH)..sup.3 Here the zero-dip shoulder effect has been corrected, and the dip-induced shoulder effect error increases with increasing dip. The apparent widening of the bed is just the geometrical effect of the longer path through the bed as dip increases. The effect on the medium induction array (IM) is similar, but is complicated by the asymmetry of the array..sup.2,3
Various authors have reported methods for correcting induction logs for dip effect. Shen and Hardman .sup.1 published charts analogous to the Thin Bed charts to correct ID and IM with traditional processing at various apparent dip angles. Howell and Fisher .sup.4 reported an inverse filter method for ID which corrected for dip, but which did not separate out other environmental effects such as shoulder effect and invasion effect. This required a plethora of filters, one for every conceivable combination of shoulder-bed-contrast invasion, contrast diameter of invasion, and dip angle. Another method, iterative inversion through forward modeling, has been used to correct for dip effect in the North Sea. .sup.5,6 This method yields accurate results, but involves many hours of computer time and considerable involvement of the log analyst.